Double-casing difusser and vertically suspended (see API610).

Typically used for low NPSHa applications, where the suction vessel is at ground level.
With this VS6 design you only need to drill a well for the installation of the pump, which will have the hydraulics at the lowest point, ensuring adequate NPSHa. If you wanted to use a cheaper horizontal pump you would need to dig a cellar a little bit deeper than the well, which would ruin your savings on the pump.
For instance: condensate pumps in a power plant, pumping saturated fluid from a condenser located at ground level.
For the calculations just make sure that you and the vendor are both refering to the same reference level. The vendor curves are normally refered to the hydraulic of the pump i.e. at the bottom and not to the suction nozzle. In this case they should give you information about the pressure drop inside the pump, specially for the suction.